1. Field of the Invention
The invention is related to a rewritable nonvolatile semiconductor storage device, such as a flash memory, and the control method thereof.
2. Description of the Related Art
NAND type flash EEPROM is formed by a plurality of memory cell transistors connected in series between the bit line and the source line, and a NAND type Nonvolatile semiconductor storage device with high integration (especially, NAND type flash EEPROM) is known.
In order to erase data for a conventional nonvolatile semiconductor storage device, a high voltage (such as 20V) is applied to the semiconductor substrate, and 0V is applied to the word line. Therefore, electrons are ejected from a floating gate with an electric charge accumulation layer consisting of poly-silicon, and its threshold value becomes lower than the erase threshold value (e.g., −3V). On the other hand, for data writing (programming), the semiconductor substrate is provided with 0V, and a control gate is applied a high voltage (such as 20V). Therefore, electrons are injected into the floating gate from the semiconductor substrate, and its threshold value becomes higher than the write threshold value (e.g., 1V). In the memory cells with the threshold value, the control gate is applied a read voltage (such as 0V) between the erase threshold value and the write threshold value, the state of the memory cell can be determined according to whether the current flowing in the memory cell or not.
In addition, in a NAND type nonvolatile semiconductor storage device, there are two kinds of following memory cells corresponding to the number of the bits that can be stored in one memory cell. (1) SLC (Single Level Cell): a memory cell is written 1 bit of data for one memory cell. (2) MLC (Multi-Level Cell): a memory cell is written multiple bits of the data for one memory cell.